Priority is claimed with respect to European Patent Application 00810350.9 filed on Apr. 20, 2000, the disclosure of which is incorporated herein by reference.
The invention relates to a gripper drum having a gripper shaft for gripping printed sheets.
Gripper shafts of this type in a sheet feeder serve to draw sheets or inserts from a stack and transfer them to an opening apparatus. A sheet feeder having two such gripper shafts is described and illustrated in, for example, Swiss patent document CH 652 103 A5 owned by the assignee of the present application. To grip the sheets, the gripper shaft is rotated about its longitudinal axis. During this rotational movement, grippers disposed on the gripper shaft grip a respective sheet on the underside of the stack, and press it against a support surface of the gripper drum. A drive having a spur gear and a closed cam, for example, is provided for rotating the gripper shaft.
During each closing process, the gripper shaft is accelerated, and its direction changes due to the play or looseness at the cam track and the toothing of the spur gear. After the turning point, the gripper shaft is inclined to rotate further, and the play is shifted to the oppositely-located outer side of the cam. If the grippers now impact the support surface of the gripper drum, the gripper shaft springs back within the play region. Notably, this causes the grippers to release thin printed sheets temporarily, after which they can no longer grip the sheets properly. This brief release may cause printed sheets to be drawn askew. Furthermore, the printed sheets can slide away from the grippers and cause an operation shutdown. Particularly in a high-output operation, the conventional gripper shaft is susceptible to disturbances of this type, which can result in a significant drop in the net output.
It is therefore an object of the invention to embody a gripper drum of the type that avoids the aforementioned disadvantages, yet can be produced inexpensively.
The object is accomplished by a gripper drum for gripping printed products which includes a drum shaft; at least one gripper disk rotatable about an axis and coupled to the drum shaft; a gripper shaft disposed on the at least one gripper disk and having a shaft body rotatable about its longitudinal axis; means for controlling rotation of the gripper shaft; a plurality of grippers disposed on the shaft body, wherein the plurality of grippers are adaptable to be rotated about the longitudinal axis of the gripper shaft for gripping the printed products; and a tensing apparatus coupled to the gripper shaft for compensating play between the controlling means and the plurality of grippers, whereby the gripper drum operates to define an opening movement and closing movement of the grippers.
In the gripper drum according to the present invention, the tensing apparatus compensates the play or looseness in the region of the toothing and the closed cam apparatus that controls the rotation of the gripper shaft. During a closing movement, a counter-moment of, for example, 6 Newton meters (Nm) is built up in the tensing apparatus. This counter-moment prevents the grippers from jumping or springing back from the opposing support surface, and can assure a gentle closing movement for grasping the printed products. The printed sheets can therefore be drawn off from the underside of a stack in a stable, secure position to result in a more reliable gripping process. This is also the case for a stack having a very high stack pressure, in which a correspondingly high gripping force is necessary. The reliable gripping process assures a higher net output and, accordingly, fewer disturbances in operation.
The gripper drum according to the present invention is especially well-suited for a sheet feeder. Other applications are also conceivable, however, in which any type of printed sheet or material can be gripped by the closing movement.
In accordance with a modification of the present invention, during each closing process, the tensing apparatus or energy accumulator includes a compensation spring that is tensed by a pivot lever, which is secured, fixed against relative rotation, to the shaft body of the gripper shaft, resulting in a particularly simple structural embodiment. The compensation spring is preferably connected to the gripper shaft and the shaft of the gripper drum. The compensation spring is automatically tensed when the gripper shaft rotates. The tension can be adjusted simply by changing the position of the pivot lever on the gripper shaft.